Many shrubs and trees have extensive root systems which reach deep into the soil profile, typically one to two meters.
Most turf and ground cover have relatively shallow roots and require more frequent application of water to replace evapotranspiration because the soil reservoir available to their root system is relatively small.
Existing practice with typical timer control systems is to provide a dual program control which enables two separate groups of valves to be timed independently for a number of days between irrigations and to water for durations independent of each other.
The problem with that type of control is that it is quite arbitrary and independent of a situation which varies from day to day as the weather changes and it has no tangible relationship with plant water use.
Experience with irrigation in practical situations using an accurate soil moisture control (such as that shown in U.S. Pat. No. 4,513,608) has demonstrated that a close relationship can be maintained automatically between the actual plant water use and the controlled irrigation water supplied.
Because of its minimum interval and for physical reasons, the most practical location for a sensor is within the root zone of most shallow rooted plants being watered by that particular control system. While this sensor location optimizes water replacement for the shallow rooted plants, those deep rooted systems requiring a deep watering less frequently suffer and excess water use can occur.